1. Field of the Invention
The present invention relates to a method and a mold for manufacturing pellets of hot-melt ink.
2. Background Art
Certain types of inkjet printers employ a so-called hot-melt ink i.e. a wax-like ink material that is solid at room temperature and has a melting point in the order of, for example, 80 to 150xc2x0 C. In the printhead of the printer, the ink is heated above its melting point, so that droplets of liquid ink can be expelled through the nozzles of the printhead. In order to obtain a high quality of the printed image, the viscosity and hence the temperature of the molten ink in the printhead should be maintained essentially constant. However, since the ink is consumed in the course of the printing process, and the ink reservoir accommodating the liquid ink within the printhead is preferably of a limited size, it is necessary to supply and melt solid ink while the printer is operating. The latent heat required for melting the ink tends to decrease the temperature in the ink reservoir. For this reason, it is desirable that the amount of solid ink supplied to the ink reservoir is precisely controlled and metered, and, to this end, it is advantageous that the ink is supplied in the form of pellets having a predetermined size and shape, e.g. in the form of small spherical pills or pellets.
Since the hot-melt ink is a thermoplastic material, the pellets having the desired shape and size can be manufactured by means of a molding process similar to injection molding processes known for manufacturing articles from thermoplastic resins. The molding process however should be adapted to the specific properties of hot melt ink, which are, in certain respects, different from those of thermoplastic synthetic resins. Since the amount of shrinkage which the hot-melt ink experiences when it is solidified is comparatively low, and since a certain amount of shrinkage can be tolerated because the final appearance of the molded ink pellets is not critical, it is not necessary to apply high locking forces for keeping the mold closed during the molding process. On the other hand, since the hot-melt ink has a relatively high melting point, it tends to solidify immediately when it comes into contact with the walls of the mold cavity. This effect and the fact that the surface of the ink pellet is somewhat tacky, even when the temperature has dropped below the melting point, increases the tendency of the pellet to adhere to the walls of the mold cavity. This makes it more difficult to reliably and reproducingly remove the molded pellet from the mold die. Especially when the upper and lower dies of the mold are symmetrical, as must be the case for example when the pellet has a spherical shape, it is not predictable whether the pellet will adhere to the upper die or to the lower die when the dies of the molds are separated. This tends to reduce the productivity of the molding process and/or necessitates the use of complex mechanisms for ejecting the molded product from the die.
It is well known that the removal of a molded product from a die can be facilitated by employing a separating agent which reduces the adherence between the molded product and the walls of the mold cavity. In this case, however, a portion of the separating agent will inevitably be dispersed or diluted in the molten material, and this is not acceptable in the case of hot-melt ink because it deteriorates the quality of the ink. For example, even minute particles of the separating agent, when dispersed in the ink, tends to clog the extremely fine nozzles of the printhead or change the ink properties such as its surface tension or crystallization point.
It is an object of the present invention to provide a method for manufacturing pellets of hot-melt ink, in which the pellets can be reliably and reproducibly withdrawn from the mold cavity.
According to the present invention, this object is achieved by a method comprising the steps of:
filling the molten ink into a mold cavity defined between a first die and a second die of a mold,
allowing the ink to cool down and to solidify in the mold cavity, and
heating at least one of the first and second dies for remelting only the surface of the ink pellet to be removed from the mold cavity.
According to the present invention, the molded pellet is separated from the wall of the mold cavity by heating at least a portion of the mold, so that a surface layer of the pellet is remelted. This can be achieved within a very short time. Thus, it is possible to remove the pellet from the mold cavity efficiently and in a well-defined manner. Since it is not necessary to employ a separating agent, the quality of the hot-melt ink will not be degraded.
When the lower die of the mold is heated before the upper and lower dies are separated, it is possible to positively release the pellet from the lower die and to withdraw it from the lower die, taking advantage of the fact that the pellet tends to adhere to the upper die which is not heated. Then, the pellet is released from the upper die by any suitable means, thereby allowing the pellet to simply drop out of the upper die. The pellets dropping out of the upper dies may be collected by any suitable collection means such as a chute which is brought in position underneath the pellets that have been withdrawn from the lower dies.
Further, it is possible to release the pellet from the upper die by heating the latter. In a preferred embodiment, the method comprises the steps of first heating the lower die, then separating the upper and lower dies with the pellets adhering to the upper die, and heating the upper die, thereby allowing the pellet to drop out. The release of the pellet from the upper die may be assisted and accelerated by blowing air into the runner hole of the upper die. As an alternative, an ejector pin may be inserted through the runner hole. In this case, the ejector pin may be arranged stationary, so it enters into the runner hole and engages the pellet adhered thereto when the upper die and the pellet are lifted from the lower die.
A mold for manufacturing pellets of hot-melt ink in accordance with the method described above comprises first and second dies defining a mold cavity, wherein at least one of the first and second dies has a wall thickness which is smaller than half the diameter of the mold cavity. If the mold cavity is not spherical, the wall thickness of the die is smaller than half the average diameter.
Due to the small wall thickness, the die has a very low heat capacity, such that the surface layer of the molded pellet can be remelted very quickly by heating the die. The small heat capacity of the die has the further advantage that the molten ink in the mold cavity can be cooled and solidified more rapidly, so that the productivity of the molding process is increased.
Preferably, both dies of the mold have a small wall thickness and hence a small heat capacity and are made of a material having a high heat conductivity, e.g. aluminium. Also stainless steel is useable if the wall thickness is small enough. In a preferred embodiment, the wall thickness of the dies is smaller than a quarter of the diameter of the mold cavity. For example, if the mold cavity is spherical and has a diameter in the order of 10 mm, the wall thickness of the dies may be 1.5 mm or less.
Rapid cooling and re-heating of the dies may be achieved in a very simple manner e.g. by blowing cold and hot air or even a liquid against the dies. A number of other heating or cooling devices can be used.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.